Útdráttur á ensku:
Weather systems developing over the North Atlantic and hitting Europe are intimately related to large-amplitude meanders of the jet stream, known as Rossby waves. Characteristic weather patterns grow in concert with the waves, and the jet stream acts as a wave guide, determining the focus of the wave activity at tropopause-level. Rossby wave energy transfers downstream rapidly, amplifying troughs and ridges.

Recent research has shown that forecast busts (where skill is much lower than usual) for Europe share a common precursor 5-6 days beforehand; a distinct Rossby wave pattern with a more prominent ridge (northwards displacement of the jet stream) across the eastern USA. The reasons for these forecast busts are not known, but it is hypothesised that the representation of diabatic (cloud and radiative heating) processes, over the USA and Atlantic, lowers the predictability in this situation.

We need new observations within the waveguide at tropopause level with sufficient vertical resolution to resolve the detailed jet stream structure and quantify the diabatic processes acting as disturbances develop. We also need to connect these “upstream” observations with a comprehensive network of observations where the downstream weather impacts occur.

An experiment tackling this inter-continental problem, NAWDEX, is taking place 16 Sep – 16 Oct 2016 based from Keflavik. The experiment involves four research aircraft equipped with lidar, radar and dropsondes for measuring high resolution cross-sections of winds, temperature and humidity. A comprehensive network of ground-based radar and lidar profiling stations are running continuously, including supersites in the UK and France, plus up to 600 additional radiosondes spanning northern high latitudes (45-65N).

The talks will outline the fundamental underpinning science regarding jet stream variability and the limitations of our understanding. The evidence motivating the NAWDEX campaign will be explained and some highlights of the campaign so far will be revealed, including the excitement of the extra-tropical transition of tropical storm Karl and its influence on the jet stream into Europe.